Ternary alloy column grid array

Abstract

Techniques and structures have been developed for providing lead-free column grid array interconnect structures. An exemplary interconnect has a body, a first joint, and a second joint, all having compositions off the eutectic composition in a ternary alloy system, the first joint having a ternary composition distinct from the body composition, and the second joint having a ternary composition distinct from the body composition and the first joint composition. The interconnect may be formed by solidifying a solder, having a Sn-poor ternary composition in the Sn—Ag—Cu alloy system, in contact with a column, having a Ag-rich Cu-deficient composition in the same system, and a bonding pad or bare substrate. A second solder, having a Sn-rich ternary composition, may be solidified in contact with the column and a second bonding pad or bare substrate. In some embodiments joints may be severed and reformed by remelting and resolidifying the lower-liquidus solder.

1. A Pb-free electrical interconnect structure comprising:
 a body having a composition off an eutectic composition in a ternary alloy system, the ternary alloy system comprising the Sn—Ag—Cu system; 
 a first joint having a ternary composition off the eutectic in the ternary alloy system, the first joint composition being distinct from the body composition; and 
 a second joint having a ternary composition off the eutectic in the ternary alloy system, the second joint composition being distinct from the body composition and the first joint composition, 
 wherein a composition of the interconnect structure varies smoothly from the first joint composition to the body composition and from the body composition to the second joint composition, said varying smoothly comprising a composition gradient of at least a constituent of the ternary alloy system and wherein the composition of the body is significantly richer in Ag than a binary Sn—Ag eutectic composition and comprises substantially no Cu, the ternary composition of the first joint is significantly richer in Ag and Cu than a ternary Sn—Ag—Cu eutectic composition, and the ternary composition of the second joint is significantly poorer in Aq and Cu than a ternary Sn—Aq—Cu eutectic composition. 
 
     
     
       2. The structure of  claim 1 , wherein the first joint composition has a liquidus temperature more than 10° C. higher than a eutectic temperature in the ternary alloy system and lower than a liquidus temperature of the body composition. 
     
     
       3. The structure of  claim 2 , wherein the second joint composition has a liquidus temperature lower than the liquidus temperature of the first joint composition. 
     
     
       4. The structure of  claim 1 , further comprising a substrate attached to the first joint. 
     
     
       5. The structure of  claim 1 , further comprising a substrate attached to the second joint. 
     
     
       6. The structure of  claim 1 , wherein the composition of the body is 80 percent by weight Sn and 20 percent by weight Ag, the ternary composition of the first joint is 91 percent by weight Sn, 7 percent by weight Ag, and 2 percent by weight Cu, and the ternary composition of the second joint is 96.5 percent by weight Sn, 3 percent by weight Ag, and 0.5 percent by weight Cu.